The present invention relates to a power supply control circuit and, more particularly, to a power supply control circuit for stopping power supply to a control unit provided between a central station and a plurality of subscriber's terminals in a cable television (CATV) system and for controlling the subscriber's terminals when in abnormal states such as when a power supply cable having both power supply and signal transmission functions to the control unit is disconnected, or when the control unit malfunctions.
As a communication system using transmission cables, an urban CATV system for allowing bidirectional communication between a subscriber's terminals and a central station is known. In CATV systems including urban CATV, the power for the control unit provided between the subscriber's terminals and the central station is paid for by the subscribers. The control unit processes subscription data and performs transmission control thereof in the polling mode from the central station. The control unit also controls data transmission from the central station to the subscriber's terminal. Thus, the control unit performs bidirectional data transmission control.
In other words, communication between the central station and the subscriber's terminals is conducted in two stages (i.e., between the central station and the control unit, and between the control unit and the subscriber's terminals). For data transmission from subscriber's terminals to the central station, transmission data is temporarily stored in the control unit. The stored data is accumulated by the central station after performing polling. When data is to be supplied from the central station to the subscriber's terminals, data is temporarily distributed to the control unit and then sent to the subscriber's terminals. Thus, in an urban CATV system, upstream and downstream data control is performed with high efficiency.
In the above urban CATV system, subscriber's terminals are connected to control units through cables. These control units are also connected to the central station through cables. Each control unit is connected to a plurality of subscriber's terminals, and the central station is connected to a plurality of control units. Thus, a large number of subscribers can be accommodated.
Television signals and transmission data from the central station is supplied to subscriber's terminals through the control unit and cable, as described above. In the subscriber's terminal, the transmission data is demodulated by a demodulator and is then fetched by a control circuit as control data. Downstream data is control data from the control unit regarding, for example, whether or not a certain subscriber is authorized, e.g., in a pay system. In accordance with the control data, the given television signal is supplied to the television receivers of authorized subscribers. As for upstream data, however, when a subscriber inputs by key pads channel information about what he wants to watch, the information is modulated by a modulator upon control by the control circuit and is then transmitted to the control unit as channel data. At this time, the control unit determines whether the channel data indicates a subscribed channel. The determination is transmitted to the modulator, and the control circuit performs the predetermined control.
In such an urban CATV system, power is supplied to the control unit not from the central station but from the subscriber's terminal side. Since the central station is relatively distant from the control unit, the control unit may be incorrectly powered and malfunction due to power tranmission loss. For this reason, power is supplied to the control unit by the subscriber's power source In this case, a voltage of about 60V is normally supplied to the control unit.
In an urban CATV system, the control unit is normally installed outdoors, and it is often installed near houses. Therefore, when the power supply cable to the control unit is disconnected, an electric shock accident may occur. When the control unit malfunctions, it may transmit noise and mix up data being sent from another control unit to the central station. Therefore, in this case, it is desirable that the power supply to malfunctioning control units be stopped immediately.